A Local Area Network (LAN) comprises a plurality of Bridging devices interconnected by links. Each Bridge contains a Filtering Database (FDB) that is referenced by frames traversing the LAN. The LAN comprises a plurality of Virtual Local Area Networks (VLANs). Each VLAN is associated with a spanning tree topology. A plurality of end-stations is attached to each VLAN. A VLAN is identified by a VLAN Identifier (VID). An end-station is identified by a Medium Access Control (MAC) address.
The number of end-stations associated with VLAN can grow large. This may be the case in virtualization environments, where a single physical server can contain multiple virtual servers. A virtual server performs the functions of a physical server but is instantiated by a logical entity known as a ‘virtual machine’ (VM). The increased number of end-stations associated with the introduction of virtual servers can impact the bandwidth consumed on links in two ways. First, if the number of <MAC, VID> pairs associated with the LAN exceeds the capacity of the FDB, FDB entries are displaced. A received frame corresponding to a displaced FDB entry is flooded (i.e., propagated on a plurality of links) on the spanning tree associated with the specified VID. The result is that link capacity consumed generally on links within a VLAN can increase significantly when the number of referenced end-stations grows beyond a threshold value related to the size of the FDB. Second, where the capacity of the FDB is sufficient to hold the plurality of referenced <MAC, VID> pairs, flooding occurs when an end-station is first attached to the VLAN or has been aged from the FDB. Such activity can be expected to increase as the total number of end-stations associated with the network grows larger. The resultant flooding increases bandwidth consumption on a plurality of links of the spanning tree associated with the VLAN. The above issues suggest problems of scale when increasing numbers of end-stations are associated with VLANs in a network such as a data center network.